This invention relates .to an optical fiber interferometer and particularly to improvements in analysis techniques using an interferometer of this type for detecting movements of or location of an object.
Optical fiber interferometers are well known and use interference techniques that detect very small differences in distance between the end of the fiber and an object. Light from a suitable light source is launched into a fiber and then through a coupler to a discharge end of the fiber. Part of the light exiting from the discharge end is reflected from the fiber to air interface and part of the light passes to the object and is reflected from the object to air interface. A portion of the second part of the light is reflected back into the fiber and passes along the fiber with the first part. These two parts interfere within the fiber in a manner which is dependent upon the phase difference between the two parts which is in turn dependent upon the distance between the end of the fiber and the object.
As is well known, this interference provides the conventional interference pattern in the form of a sinusoidal variation in light intensity as the distance between the object and the fiber is changed. In order to effect a proper analysis of the distance, it is normally necessary to maintain the distance within a relatively narrow band where the sinusoidal wave form is substantially linear, that is well spaced from the peaks. It will of course be appreciated that this detection technique cannot distinguish between a change in distance of X and a change in direction of ##EQU1## where .lambda. is the wave length of the light.
Normally the analysis technique simply involves a comparison of the output signal, as an electrical voltage measured by a photo diode, at one location of the object and the output signal at a second location of the object. Provided the movement of the object between the two location lies substantially in the linear range of the sinusoidal wave pattern, these changes in output signal are substantially proportional to the change in distance.
However these analysis techniques are relatively limited leading to a limitation in the effectiveness of the interferometer.
It is one object of the present invention, therefore, to provide an improved method of detecting the location of an object using fiber interferometer techniques.
According to the invention, therefore, there is provided a method of detecting a location of an object comprising providing an optical fiber having a light inlet end and a light discharge end, providing a light source at the light inlet end for transmitting light along the fiber, said light source having a primary wave length .lambda., locating the light discharge end adjacent to but spaced from the object to define a fiber to air interface at the discharge end and an air to object interface at the object such that light transmitted along the optical fiber from the inlet end to the discharge end has a first part which is reflected from the fiber to air interface back along the fiber and a second part which is reflected from the air to object interface back into and along the fiber, the distance D between the object and the discharge end being arranged such that the first part and the second part interfere in a manner depending upon the phase difference therebetween, detecting with respect to time variations in the quantity of light reflected, and cyclically varying the optical path length between the object and the discharge end of the fiber, the cyclical variation having a predetermined frequency and a predetermined amplitude, and analyzing the changes in the quantity of light reflected with respect to time to determine information concerning the location of the object.
The technique of the present invention can therefore involve applying a cyclical variation or sinusoidal dither to the optical path length between the sample and the discharge end of the fiber with that dither having a carefully controlled frequency and amplitude. This dither can be obtained by physical movement of one of the sample and the discharge end or by electro-optical means or by other means. This allows in general terms the output from the photo diode of the interferometer to be analyzed as an alternating signal varying over time rather than as a simple comparison of absolute values as used in the conventional techniques.
This variation as an alternating signal over time can be most effectively analyzed using AC circuitry techniques to detect various aspects of the wave form so generated as described in detail hereinafter.
One embodiment of the invention will now be described in conjunction with the accompanying drawings in which: